Airlines charge to check in each bag. In fact, the rate goes up depending on how many you check in. My wife, Zaheen, flew out to Austin, TX yesterday on Southwest Airlines and it turns out that they don’t charge for the first two bags. That’s how it use to be with all the airlines but seems like they are the only ones around that do not charge for the first two bags still.
Here are the airlines that I know of that charge
American Airline, America West/US Airways, Delta:
$15 for the first checked piece. ($30 on a round trip flight)
$25 for the second checked piece. ($25 + $15 = $40 x 2 = $80 on a round trip flight)
Oh and did I mention Southwest still serves snacks =)